Stefan Leupold (Uppsala, Sweden)

Exploring matter at the femtometer scale: Model-independent calculations of electromagnetic form factors

At high energies, the (transition) form factors of hadrons are most sensitive to the respective minimal quark content. At low energies, however, the form factors are dominated by universal features related to pion physics. Only these Goldstone bosons can carry information over large distances. Dispersion theory is used to obtain a model-independent representation of vector-isovector baryon (transition) form factors at low energies. The ingredients are pion-baryon scattering amplitudes and the well-measured pion vector form factor. The latter is related to the p-wave pion phase shift, which contains as its most prominent feature the information about the rho meson. As a consequence, the dispersive framework leads to a model-independent version of vector-meson dominance. In the present scheme, motivated by chiral perturbation theory, the pion-baryon amplitudes are constructed from baryon-exchange diagrams providing the long-range aspects and subtraction constants for the short-distance physics. Also here the measured p-wave pion phase shift is utilized to account for the strong interactions between the pions. The baryon exchange diagrams are typically not included in models of vector-meson dominance and also not in quark models, in clear distinction to the scheme presented here. Up to now, the subtraction constants constitute free parameters and are determined by fits to experimental or lattice data. We apply this dispersive low-energy scheme to 1. the form factors of the nucleon, with focus on the dependence on both the virtuality and the quark mass [1,2]; 2. the transition form factors of nucleon to Delta(1232) [3]; 3. the transition form factors of nucleon to N*(1520) [4]. An outlook is provided about extensions and applications to meson form factors relevant for the magnetic moment of the muon, weak form factor relevant for neutrino-matter scattering, and hadronic input for electromagnetic radiation from hot/dense strongly interacting matter.

[1] Stefan Leupold, Eur.Phys.J.A 54 (2018) 1, 1
[2] Fernando Alvarado, Di An, Luis Alvarez-Ruso, Stefan Leupold, Phys.Rev.D 108 (2023) 11, 114021
[3] Moh Moh Aung, Stefan Leupold, Elisabetta Perotti, Yupeng Yan, arXiv 2401.17756 [hep-ph]
[4] Di An, Stefan Leupold, in preparation